1. Technical Field of the Invention
The present invention relates generally to bearings and automotive accessories. More particularly, the invention relates to a rolling bearing assembly for use in an automotive accessory, which includes a magnet to prevent the surfaces of rolling elements and the raceway surfaces of inner and outer rings thereof from brittle flaking.
2. Description of the Related Art
In recent years, rolling bearings of automotive accessories, such as an alternator and an air conditioning compressor driven by an engine in an automobile, have come to be used under severe operating conditions (for example, high temperature and vibration), resulting in a new type of bearing damage, called “brittle flaking”.
Brittle flaking may occur at any area of the surfaces of the rolling elements and the raceway surfaces of the inner and outer rings of the rolling bearing. Further, brittle flaking has a feature that the time period from the start to finish thereof is very short, for example, only about 0.1 to 1% of that of a general rolling contact fatigue.
Up to now, the mechanism of brittle flaking has been not made clear. Accordingly, only temporary expedients that are not based on sound scientific grounds have been employed to solve the problem of brittle flaking, in other words, no fundamental solution to the problem has been provided.
According to the results of a rolling bearing reliability test, among the accessories of an automobile, occurrence rate of brittle flaking was highest in the alternator. As is well known in the art, the alternator had the highest speed increasing ratio with respect to the engine and a large rotational inertia, and thus had the largest equivalent inertia which is proportional to the second power of the speed increasing ratio. Further, occurrence rate of brittle flaking was high in the accessories when those were driven by the engine via a Poly-V belt, with which the belt tension was set tight as is well known in the art. Furthermore, occurrence rate of brittle flaking was high in the accessories when there was provided an autotensioner in the belt drive system to prevent slack of the belt via which the engine drove the accessories.
One theory explains, based on the fact that the hydrogen content in grease included in rolling bearings damaged due to brittle flaking is high, the mechanism of brittle flaking such that the hydrogen generated due to decomposition of the grease diffuses into the rolling elements and the outer ring, thereby causing the surfaces thereof to flake. (With regard to the theory, a further reference can be made to: 1) Minoru Umemoto, Sanyo Technical Report, Vol. 11, No. 1 (2004), “Nanocrystallization by Heavy Deformation and White Etching Area in Bearing Steels”; 2) K. Tamada and H. Tanaka, Wear 199 (1996), “Occurrence of Brittle Flaking on Bearings Used for Automotive Electrical Instruments and Auxiliary Devices”; and 3) Japanese Patent First Publication No. 2002-227854.)
Further, according to the theory, a new type of grease has been developed which includes an additive to form electrical insulation films on raceway surfaces of rolling bearings, thereby preventing brittle flaking from occurring. By means of the new grease, a certain decrease in occurrence rate of brittle flaking has been achieved; however, it is still impossible to completely prevent occurrence of brittle flaking only by the help of the grease. Indeed, in some cases, brittle flaking occurred even with the insulation films formed on the raceway surfaces of the rolling bearings. Moreover, on the surfaces damaged by brittle flaking, the evidence of a plastic deformation was found.
Accordingly, though grease does have influence on the occurrence of brittle flaking, it may not be effective to solve the problem of brittle flaking by developing a newer type of grease aiming to form insulation films on raceway surfaces of rolling bearings.